Download Media Preparation & Sterilization

General Microbiology Laboratory
Media Preparation & Sterilization
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
Medium : a nutrient blend used to support microbial growth.
There are three physical forms of media, broth, solid, and
semisolid.
 Solid media are more versatile in their usage.
• Promote surface growth
• Used to isolate pure cultures
• Ideal for culture storage
• Helpful in the observation of biochemical reactions
• Used to make slants, deeps, and plates (named by
medium)
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
A medium is sterilized (living organisms removed) before
usage in the lab.
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Sterilization methods include; autoclaving, dry-heat,
filtration, UV exposure and ethylene oxide.
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Culture: Is part of specimen grown in culture media.
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Culture Media: is a medium (liquid or solid) that contains
nutrients to grow bacteria in vitro. Because sometimes we
cannot identify with microscopical examination directly, and
sometimes we do culture for antibiotic sensitivity testing.
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Properties of Media:
Support the growth of the bacteria.
Should be nutritive (contains the required amount of
nutrients).
Suitable pH (neutral to slightly alkaline 7.3-7.4).
Suitable temperature, and suitable atmosphere.
(Bacteria grow at 370C)
Note: media are sterilized by autoclaving at 1210C and 2
atmosphere for 15-20 minutes. With the autoclave, all bacteria,
fungi, viruses, and spores are destroyed. Some media can’t be
sterilized by autoclaving because they contain eggs or
carbohydrates .
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Forms of culture
Solid (agar): Is Broth plus agar (seaweed).
Are prepared by adding a solidifying agent (agar 1.5 3%).Prepared mainly in Petri dishes, but also in tubes
and slopes. After growth the bacterial colonies are
visible. e.g. blood agar, chocolate agar, MacConkey
agar.
Semisolid agar (soft agar): Contains small amounts of
agar (0.5-0.7%). Used to check for motility and also
used as a transport media for fragile organisms. Can
have semisolid agar in Petri dishes or in tubes. In
tubes it is usually slanted to increase surface area.
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Liquid (Broth): Mostly used for biochemical tests
(blood culture, Broth culture). Growth of bacteria is
shown by turbidity in medium. e.g. Nutrient broth,
Selenite F broth, alkaline peptone water.
Properties of agar:
• Some what like gelatin.
• It melts at 970C and solidifies at 370C.
• Comes as sold powder and then you add water to it.
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Types of Culture Media
Simple (basal, ordinary): Culture Media: are
media that contain the basic nutrients (growth
factors) that support the growth of bacteria
without special nutrients, and they are used as
basis of enriched media. E.g. Nutrient broth,
nutrient agar, peptone water. They are for the
growth of non-fastidious organisms like E.
coli.
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Enriched Culture Media: are media that are enriched
with: Whole blood e.g. blood agar. Lysed blood
(heated to 80C) e.g. Chocolate agar.
Selective Media: it is a media, which contains
substances that prevent or slow the growth of
microorganisms other than the bacteria for which the
media is prepared for. For example
• TSI (triple sugar iron agar): slanted tube.
• EMB (Eosin Methylene blue): enteric isolation media.
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Differential Media (indicators): Contains indicators,
dyes, etc, to differentiate microorganisms. E.g.
MacConkey agar, which contains neutral red (pH
indicator) and is used to differentiate lactose
fermenter and non-lactose fermenter. (E.g. E. coli and
Salmonella).
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Common media used in Microbiology Laboratory
Chocolate Agar: blood agar prepared by heating
blood to 95C until medium becomes brown or
chocolate in color heating the blood releases broth X
and V growth factors and also destroys the inhibitors
of V factor. These factors are required for the growth
of most species of Haemophilus and also Neisseria
gonorrhoear.
MacConkey Agar: an inhibitory and differential
medium used to distinguish lactose- fermenting
Gram- negative organism from non fermentation.
Crystal violet, bile salts and neutral red are inhibitor
agent. neutral red is the PH indicator.
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 Mannitol Salt Agar ( MSA ): for selective isolation for
coagulase positive, mannitol-fermenting staphylococcus.
• Mannitol fermentation by pathogenic staphylococci is
indicated by a yellow halo surrounding the colonies.Sodium
chloride is the inhibitor agent.Phenol red is the PH indicator.
 Mueller Hinton Agar: rich medium that support the growth
of most microorganism. It is commonly used for antibiotic
susceptibility testing: disk diffusion antibiotic susceptibility;
antibiotic serum level measurements; MBC determination.
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 Salmonella Shigella ( SS ) Agar : isolation and differential
medium for pathogenic Gram-negative bacilli in particular,
Salmonella and Shigella. Inhibitor for Coliforms.
 Triple Sugar Iron Agar (TSI): this a key medium for use in
beginning the identification of a Gram- negative bacilli of the
enteric group. It contains glucose (0.1% ), Lactose (1%),
sucrose(1%). And peptone (2%) as nutritional sources. Sodium
Thiosulfate serves as the electron receptor for reduction of
sulfur and production of H2S. Detects fermentation of sucrose,
lactose, glucose, as well as production of hydrogen sulfide and
/or gas . Phenol red is the PH indicator; ferric ammonium
citrate is H2S indicator.
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End of Lecture
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